This invention relates to improvements in apparatus for the protection of electrical power lines.
Several fault protection techniques are known for power lines (including electrical cables). They each have several common features and yet can be divided into at least two distinct groups.
In a general fault protection scheme, a fault protection circuit which includes a circuit breaker is provided in the electrical line. The breaker has a normally closed contact which connects two portions of the line when no fault is present. When a fault is detected, the contact is opened to isolate one part of the line from the other. A trip relay is provided which detects the current or voltage flowing in the line to provide an output signal. If the output signal is indicative of a fault on the line, the circuit breaker is opened. The exact construction of this fundamental building block of fault protection systems and its variants is well known and will not be described further herein.
The known fault protection schemes differ in the way in which the output signal is processed to control the opening of the circuit breaker. Two broad categories of scheme are known as non-unit protection and unit protection. Each has its own limitations.
An example of a non-unit fault protection scheme is the distance protection scheme. In this scheme a single circuit breaker protects a length of line section, for example over the ends xe2x80x98Sxe2x80x99 and xe2x80x98Rxe2x80x99 as shown in FIG. 1(a).
A single measurement device (the xe2x80x9crelayxe2x80x9d) provided at a measurement point xe2x80x9cM1xe2x80x9d adjacent the circuit breaker provides an output signal indicative of the signal on the line. From the single measurement, the protected portion of line covered by the circuit breaker is divided into a number of zones, typically two zones. The first zone, zone 1, may cover up to 80% of the line section from the circuit breaker. This is referred to as the reach point and is shown in FIG. 1(a). Zone 2 is normally arranged to cover 50% of the shortest adjacent line beyond the protected line. Again this is shown in FIG. 1(a). Since the impendance of a line is proportional to its length, the zone 1 can be defined using a relay capsule of measuring the impedance of the line up to a certain point. This is known in the art as a distance relay. It is designed to operate its circuit breaker only for faults occurring between the relay location and its reach point.
For a fault occurring within zone 1, say at a point xe2x80x98F1xe2x80x99 the fault can be detected by monitoring the output signal from the relay. The circuit breaker is adapted to be opened instantly upon detecting the presence of a fault in zone 1.
For a fault occurring within zone 2 and outside zone 1, say at a point xe2x80x98F2xe2x80x99, the overall response time of the circuit breaker is chosen to be significantly longer. This provides enough time for an adjacent (remote) circuit breaker on the line nearer to the zone 2 fault to open, hopefully clearing the fault without the need to open the local circuit breaker. In the example, if the second circuit breaker employs a two zone protection scheme, F2 will be within its zone 1 area causing an instant trip.
A problem with the non-unit protection technique described above is that the boundaries of the zones are not well defined, especially zone 2.
Unit protection techniques, on the other hand, have a clear defined protection zone as shown in FIG. 1(b). The boundaries are user defined by using information from the local relay and also from a remote relay at the other end of the protected section of line. The system effectively makes two point measurements at points xe2x80x9cM1xe2x80x9d and M2xe2x80x9d, i.e. from each end of the protected section.
A communication link is used to transmit information about the system condition from one end of the protected section to the other. With this connection, the relays at both ends can be arranged to trip instantly to isolate the protected portion of the line when a fault occurs on the protected portion, say at xe2x80x9cF1xe2x80x9d. For a fault outside of the protected portion, the circuit breakers may both be prevented from opening, for instance for a fault at the point xe2x80x9cF2xe2x80x9d.
Whilst the unit protection scheme provides more defined zones than the non-unit protection scheme by allowing accurate location of the fault position, it is expensive to implement and the communication links are undesirable.
In a modification of the two schemes, it is known to combine the distance protection with the communication links in what is known as a xe2x80x9cpermissive undereach schemexe2x80x9d. In this scheme, shown schematically in FIG. 1(c), a protection relay is provided at each end of a protected portion of a line. Each relay is adapted to make an instant fault trip of its associated circuit breaker for a fault occurring within a zone 1 distance along the line. This zone 1 typically defines 80% of the line section length. For example, for a fault at position xe2x80x9cF1xe2x80x9d in the Figure, both circuit breakers will be opened instantly. However, for a fault occurring outside of zone 1 at one relay, only one circuit breaker will be instantly tripped. A signal passed along a communical link between the two relays can be used to send information from the relay that has instantly tripped to the other relay. Only if an appropriate signal is received will the other circuit breaker be opened as the fault lies outside its protected zone 1 section. Again, this scheme suffers from the need for a communication link between the relays.
An object of the invention is to provide a fault protection scheme which at least partially alleviates the problems present in prior art systems incorporating communication links.
In accordance with a first aspect of our invention a section of a line is protected by providing a local fault protection assembly at one end of the section distal from at least one remote fault protection assembly at the other side of the section, each assembly comprising a circuit breaker, a line signal measurement means adapted to produce an output signal indicative of the voltage or current on the line, and a signal processing means adapted to process the output signal to produce a fault signal in the event that the output signal indicates the presence of a fault in the line, the method comprising the steps of monitoring the output signal from the signal measurement means at the local circuit breaker to detect the presence of a fault within a first zone extending over at least a part of the section from the local breaker or outside of this first zone, and in the event that the output signal is indicative of a fault within the first zone substantially instantly opening the local circuit breaker, and in the event that the fault is outside the first zone detecting the operation of the remote circuit breaker by monitoring the output signal from the local signal measurement means, monitoring the output signal to determine the condition of the section of line after the remote circuit breaker has been operated, and operating the local circuit breaker in the event that the fault condition meets a predetermined criteria.
By a part of the protected section, we mean 50% or 60% of the length of the section. It may be more than this, say 90%, or less, say 10%.
The invention therefore provides a method in which unit protection is implemented without the need for a remote communication link between the local and remote circuit breaker. This is made possible by suitable processing of the output signal.
By operation of the remote circuit breaker we may mean opening or closing of a trip contact of the remote circuit breaker provided in the line. By operating the local circuit breaker, we mean opening or closing the circuit breaker.
The method may comprise the additional step of substantially instantly opening the local circuit breaker regardless of whether a fault is detected which is within the first zone or outside the first zone. If the output signal from the signal measurement means indicates that the fault has not been cleared after the operation of the remote circuit breaker has been detected, the local circuit breaker may be inhibited from reclosing. If the fault is cleared, the local circuit breaker may be reclosed. This is known as the xe2x80x9cinstant operation modexe2x80x9d.
An advantage of the instant operation mode is that it offers increased protection by automatically opening the local circuit breaker to isolate the line section for any detected fault condition. The detection of the operation of the remote breaker combined with the detection of the fault condition after it (the remote breaker) has operated, allows rapid clearing of the fault.
In an alternative, if a fault is detected by monitoring the output signal which is not within the first zone, the circuit breaker cannot be opened instantly. Instead it may be opened after a predetermined time delay. In the event that the operation of the remote circuit breaker is detected by monitoring the output signal and indicates that the fault on the line persists after this operation, the local circuit breaker may be opened at an accelerated time without waiting for the predetermined delay. If the fault has cleared then the local circuit breaker may be inhibited from opening at all. This is known as the xe2x80x9cdelayed operation modexe2x80x9d. Operation in either the instant or delayed mode may be selected during installation of the apparatus.
In a yet further modification, the fault protection may be adapted to choose either instant operation or delayed operation adaptively in response to the type of fault which occurs.
It will be therefore be understood that the invention lies, at least in part, in monitoring the output signal from a single measurement sensor at a local circuit breaker to determine the effect of the operation of a remote circuit breaker on the fault signal present on the line to decide how to operate the local circuit breaker.
The remote fault protection assembly may also be adapted to operate in the same way as the local fault protection assembly described hereinbefore. Thus, any circuit breaker on the line may operate as a xe2x80x9clocalxe2x80x9d breaker, with adjacent breakers on the line being xe2x80x9cremotexe2x80x9d breakers.
The operation of the remote circuit breaker may be detected by monitoring the output signal over time to detect changes in value indicative of opening of a remote breaker. A knowledge of the time delay between detecting a fault and the expected closure or opening of adjacent remote breaks can be used to help identify the remote operation.
The output signal may comprise three output sub-signals, one sub-signal per phase for a three phase line. Any such three phase system may be represented as the sum of three balanced three-phase components; positive sequence having the same phase rotation as the original system, negative sequence having the opposite phase rotation, and zero sequence having no phase rotation. In the event of an unbalanced or asymmetrical fault, negative and zero sequence quantities will be generated. These components can be observed in the output sub-signals and used to identify an initial fault.
Operation of a remote breaker can be detected by looking for changes in the sub-signals after the initial fault has been detected.
On a three-phase line, a fault on the line may be detected by looking for an output signal indicative of an unbalanced current or voltage. A system is unbalanced when three phase currents and/or voltage signals are not of equal amplitude and/or the phase differences between successive signals are unequal. It is expected that most fault conditions will provide unbalanced or transient signals. The line signal will normally be balanced in a no fault operating condition.
By line signal we mean the current or voltage on the line. This may typically be a sinusoidal three phase voltage or current signal.
In the event of a balanced three phase to earth fault, only a positive sequence component will be produced in the output sub-signals. This can be used to detect the presence of such a fault condition. A change in this positive sequence quantity can be used to detect the operation of a remote breaker operation.
The positive and/or negative and/or zero sequence components may be detected using a real-time processing algorithm.
One or more ratio signals may be calculated from the RMS values of the sequence components. A first ratio may be used to indicate an asymmetrical fault. The second ratio may be used to identify a three phase symmetrical fault.
The first zone may be defined by known distance protection techniques. It may cover approximately 80% of the length of the protected portion of line.
The output sub-signals may be indicative of the voltage or the current in the phases of the line.
The current sensing devices may comprise relays such as electromechanical relays or static relays and may be combined with digital or analogue electronic processing means.
According to a further aspect, the invention provides apparatus for protecting a section of a line comprising a local fault protection assembly provided at one end of the section distal from at least one remote fault protection assembly provided at the other side of the section, each assembly comprising a circuit breaker, a line signal measurement means adapted to produce an output signal indicative of the voltage or current on the line, and a signal processing means adapted to process the output signal to produce a fault signal in the event that the output signal indicates the presence of a fault in the line, in which the apparatus includes means for monitoring the output signal from the signal measurement means at the local circuit breaker to detect the presence of a fault within a first zone extending over at least a part of the section from the local breaker or outside of this first zone, and means for controlling the circuit breaker so that in the event that the output signal is indicative of a fault within the first zone the controlling means is adapted to substantially instantly open the local circuit breaker, and in the event that the fault is outside the first zone the monitoring means is adapted to detect the operation of the remote circuit breaker by monitoring the output signal from the local signal measurement means, and the monitoring means is further adapted to monitor the output signal to determine the condition of the section of line after the remote circuit breaker has been operated, the control means being adapted to operate the local circuit breaker in the event that the fault condition meets a predetermined criteria.
The further optional and preferred feature of the apparatus can be determined from the description of the method hereinbefore.